The invention relates to a brake for a hydraulic vehicle brake system according to the pre-characterizing clause of claim 1, to a sealing ring for such a brake, to a method for producing such a sealing ring and to a method for coating a brake piston.
Hydraulic vehicle disk brakes have at least one piston/cylinder unit, by means of which an associated brake lining, which consists of a lining carrier plate and of a friction lining fastened thereto, can be pressed against the brake disk, in order to brake the rotational movement of a wheel connected to the brake disk. The brake lining is pressed onto the brake disk as a result of the piston being displaced in the direction of the brake disk by means of hydraulic pressure. So that this action functions perfectly, the piston must be sealed off relative to the bore or cylinder, respectively, in which said piston is received. This fluid sealing is conventionally achieved by means of an annular elastomeric seal which is normally held in a groove of the bore or cylinder, respectively, and the radially inner circumferential surface of which bears on the outer surface of the piston. The elastomeric sealing ring mostly has a square or rectangular cross section.
During braking, the piston moves in the direction of the brake disk by means of hydraulic pressure and the elastomeric sealing ring is somewhat taken up by the displacing piston and, as a result of this and of the hydraulic pressure, is somewhat deformed. When the brake is released, that is to say when the hydraulic pressure ceases, the deformed elastomeric sealing ring exerts a return force on the piston and entrains the latter a little distance away from the brake disk. This behavior is plainly desirable, since it resets the brake piston and ensures that, after braking has ended, a so-called lifting clearance is established between the friction lining and the brake disk. Reference is made, in this respect, to German Patent 1,600,008 which describes this behavior in detail. In order to assist the above-described behavior of the elastomeric sealing ring, often even the groove side wall which is located on the brake-lining side and in which the sealing ring is held is slanted, that is to say the groove cross section widens from the groove bottom (see, in this respect, French Patent 1,504,679). So that the sealing ring can reset the brake piston, some adhesion between the cooperating surfaces of the sealing ring and of the brake piston is necessary. If the sliding properties between these surfaces were too good, the piston would be prevented from being reset, since it would be slideably displaced relative to the sealing ring almost immediately, with the result that there would no longer be any deformation of the sealing ring and, consequently, also any resetting force.
On the other hand, under certain circumstances, it is highly desirable for the brake piston to slide relative to the sealing ring. This is the case, for example, when the brake is being assembled. Here, after the sealing ring has been inserted into the cylinder wall groove receiving it, the brake piston has to be pushed into the cylinder. If the piston and sealing ring were unable to slide relative to one another, it would be highly likely that the sealing ring would be twisted in the groove or that it would be damaged and partly sheared off. In certain operating states of the brake, too, however, it is necessary for the brake piston to slide relative to the sealing ring, for example when the thickness of the friction lining decreases due to wear, so that the brake piston has to be displaced correspondingly further in the direction of the brake disk in order for the friction lining to abut the brake disk. Under such circumstances, the deformability of the elastomeric sealing ring is exceeded, and the piston has to slide relative to the sealing ring in order to assume a new position relative to the latter. Sliding of the brake piston relative to the sealing ring may become necessary even during very sharp braking operations, since then, due to the high forces, the brake housing is widened, the material of the friction lining is compressed, etc., so that a correspondingly longer displacement travel of the brake piston has to be produced.
In order to promote the sliding properties between a brake piston and the elastomeric sealing ring sealing it off, it has long been conventional to soak the sealing ring in an assembling fluid prior to assembly. Sealing rings soaked in this way are inserted into the groove in the cylinder wall of the piston/cylinder unit, and the associated piston can then be pushed readily into the cylinder, without damaging the sealing ring.
However, the use of an assembling fluid also leads to a series of problems. Thus, it is necessary to make sure that the sealing rings do not dwell too long in the assembling fluid. Also, constituents of the sealing rings are dissolved in the assembling fluid, so that the latter has to be changed from time to time. Furthermore, the sealing rings soaked in the assembling fluid must be further processed, that is to say assembled, within a specific period of time. Moreover, when a vehicle brake system is filled with hydraulic fluid for the first time, the assembling fluid may lead to undesirable secondary effects, such as, for example, foaming.
It is the object or the invention to provide a brake, during the assembly of which the use of assembling fluid may be dispensed with and in which the sliding properties between the brake piston and the elastomeric sealing ring are such that good sliding of the brake piston relative to the sealing ring becomes possible, but without losing the desired resetting property of the sealing ring.
Proceeding from a generic brake, this object is achieved, according to the invention, in that the sealing ring and/or at least the outer surface of the brake piston is coated with a lacquer which contains an organic binder and a solid lubricant. Wax with resins is preferably used as the organic binder. The organic binder serves as a matrix for the solid lubricant and binds the latter.
Preferably, the solid lubricant is one which is based on polytetrafluoroethylene, graphite or molybdenum disulfide.
Particularly preferably, when the brake piston is coated with lacquer, the transitional region between the outer surface and the piston head of the brake piston is also coated with the lacquer.
The lacquer layer applied to the brake piston is preferably baked, in order to increase its stability and, in particular, its abrasion resistance. During baking, it is necessary to ensure that the lacquer layer keeps the desired surface properties, that is to say, in particular, does not become too rough.
The thickness of the lacquer layer is advantageously about 5 xcexcm to about 15 xcexcm.
The object mentioned at the beginning is also achieved by means of an elastomeric sealing ring for a vehicle disk brake, said elastomeric sealing ring being coated with a lacquer which contains an organic binder and a solid lubricant. The solid lubricant is preferably based on polytetrafluoroethylene, graphite or molybdenum disulfide. Preferably, the entire sealing ring is coated with the lacquer.
Advantageously, the sealing surface of the sealing ring has pocket-like depressions before the lacquer layer is applied. On the ready-coated sealing ring, this achieves, on the sealing surface of the latter, a solid lubricant repository effect which ensures constant properties of the sealing ring according to the invention over a long period of time.
A preferred method for producing sealing rings coated according to the invention comprises the introduction of elastomeric sealing rings into a drum and the all-round wetting of the introduced sealing rings with a liquid lacquer, which contains an organic binder and a solid lubricant, as a result of the rotation of the drum, the liquid lacquer being located in the drum. Alternatively, the liquid lacquer may also be sprayed into the drum during the rotation of the latter, in order to wet the sealing rings with the lacquer. The production method according to the invention comprises, furthermore, removal of the excess lacquer from the sealing rings by rapidly rotating the drum, draining the excess liquid lacquer out of the drum and drying the sealing rings coated with lacquer. Drying preferably takes place at a temperature of 60 to 70xc2x0 C.
Waxes with resins are preferably used as the organic binder for the solid lubricant.
Preferably, the liquid lacquer used in the method according to the invention contains a mixture of ester and alcohol as solvent. It has proved beneficial if the liquid lacquer contains up to 20% by weight of organic binder with solid lubricant and about 80% by weight of solvent. As already stated previously, the solid lubricant may be a solid lubricant based on polytetrafluoroethylene, graphite or molybdenum disulfide.
Alternatively, the sealing rings may be coated with lacquer by being laid onto a belt, preferably a conveyor belt, and sprayed with lacquer on this belt. However, this coating method makes it necessary for the sealing rings to be turned over. The sprayed sealing rings may be led on the belt through a belt dryer for drying.
It has proved beneficial, before coating with lacquer, to roughen that sealing surface of each sealing ring which cooperates with the brake piston. Pocket-like depressions are thereby produced in the sealing surface of the sealing ring, and, during coating with lacquer, repositories of solid lubricant are formed in these depressions which ensures constant properties of the sealing ring according to the invention over a long period of time. Alternatively, the pocket-like depressions may be made as early as during the production of the blanks or semi-finished articles for the sealing rings, in particular by the use of a mold core having a correspondingly designed surface which is reflected in the elastomeric mass when the latter solidifies.
If the brake piston is to be coated with lacquer, according to the invention the lacquer is preferably applied to the brake piston by spraying. The lacquer layer applied to the brake piston is then baked in order to increase its stability and abrasion resistance, wherein the applied lacquer may be dried and baked simultaneously. Preferably, the lacquer layer is baked for half an hour at a temperature of 180xc2x0 C.
The brake according to the invention provides excellent resetting of the brake piston under a multiplicity of different operating conditions. Thus, in conventional brakes, there is, for example, the problem that the brake piston is no longer reset sufficiently after particularly sharp braking. As mentioned, in the event of sharp braking operations, the brake piston has to slip relative to the sealing ring in order to execute the longer displacement travel required. However, the sealing ring is not capable of resetting the brake piston by the same amount. The result of this is that, even when the brake is released, the friction lining still rubs against the brake disk, the consequence of this being, in addition to increased fuel consumption, that the brake system may overheat and therefore fail.
This problem virtually no longer arises in brakes according to the invention. Although, here too, the sealing ring is not capable of resetting the brake piston over a corresponding distance after sharp braking operations, test results have shown that, when a sealing ring coated according to the invention is used, there is nevertheless a sufficiently large lifting clearance after sharp braking operations, that is to say the friction lining no longer rubs against the brake disk. This is attributable to a good sliding behavior between the sealing ring and the outer surface of the brake piston, but without this impairing the resetting capacity of the sealing ring according to the invention during normal, relatively smooth braking operations.
Brakes according to the invention and sealing rings equipped according to the invention are therefore particularly suitable for coming generations of motor vehicles, in which the fuel saving is of primary significance and where it is therefore important, under all circumstances, to prevent the brake linings from rubbing against the brake disks or other members to be braked. Extrapolations have shown that, by the decrease in the residual rubbing moments, the fuel consumption of a passenger vehicle can be reduced by up to half a liter over one hundred kilometers. In order to achieve a fuel saving of the same order of magnitude by a decrease in the vehicle weight, the latter must be reduced by about 100 kg, which is achievable only with considerable expenditure in terms of construction and materials.
An embodiment of a vehicle brake according to the invention and test data obtained from it are explained in more detail below.